It is conventional practice in the manufacture of semiconductor devices and, in particular, in the manufacture of integrated circuit devices, to form an ordered series of layers of different types of materials over the surface of a suitable substrate. After each individual layer of material is applied, it is generally patterned by forming a film of photoresist over the layer, selectively exposing areas of the film of photoresist, and developing the exposed film of photoresist with a suitable solvent to form a patterned photoresist mask over the layer of material. The exposed areas of the underlying layer of material are then selectively subjected to various treatments such as etching, ion implantation and the like. The film of photoresist remaining on the layer of material is then stripped from the surface of the layer of materials with a solvent for the photoresist or by etching away the film of photoresist with a plasma.
An alternate method has been suggested for forming a patterned glass layer wherein a layer of a mixture comprised of glass frit and a photoresist composition is applied over the surface of a substrate. The resulting layer is then selectively exposed. After exposure, the layer of the mixture is developed with a solvent which selectively removes the photoresist and the associated glass frit from predetermined areas of the layer of the mixture defined in the exposure step. The remainder of the layer of material is then fired at an elevated temperature to burn out the photoresist and to fuse the glass frit.
In the manufacture of semiconductor devices in general and particularly in the manufacture of the more recently developed miniature microwave integrated circuit devices, it is extremely important that the layers of material comprising the devices be essentially defect-free and that the surfaces of the layers be as smooth as possible.
In this regard, it should be noted that the microwave integrated circuits are typically manufactured by initially forming a substrate from an insulative material such as alumina, beryllium oxide or the like. The substrates typically included connectors such as plated-through holes or veins of conductive material at predetermined locations for making connections of the completed microwave device with a ground plane or the like. The surfaces of the substrates are highly polished. However, despite the careful polishing methods employed, the surface of the polished substrates still have pits formed in them, some of which are believed to be caused by tiny crystals of material being broken out from the surface of the substrate during polishing. It has been found that these minor defects in the substrate interfere with the proper operation of the completed microwave devices. To correct the defective surfaces, it has been suggested to form an insulative planarizing layer having an extremely smooth, defect-free surface except for a predetermined pattern of apertures defined therein corresponding to the positions of the conductors over the surface of the substrate.
Attempts have been made to form the patterned, smooth layer on the surface of the polished substrates using the methods generally outlined above. It has been suggested to use polyimide polymers to form the smooth layer but these polymers tend to blister and have excessively rough surfaces. Attempts were also made to form a glass layer using a mixture of glass frit and a photoresist composition. The mixture can be applied satisfactorily and photolithographically patterned; however, when the developed layer of the mixture of glass frit and the photoresist composition is fired, the photoresist is not completely removed with noticeable amounts of partially decomposed photoresist composition remaining in the fused glass frit. On re-heating the coated substrate during subsequent firings of other layers, the residual organic materials from the photoresist composition decomposed and formed gaseous materials within the fused glass layer. The gases that developed frequently caused blisters, voids and generally made the surface of the glass layer excessively rough so as to be unsuitable for a microwave device.
It would be highly desirable if a method could be provided which could be employed to form smooth defect-free patterned glass layers over the surface of substrates used for semiconductor devices and particularly microwave devices.